As the desire for bandwidth increases, the technologies supporting the additional bandwidth must too expand. With this expansion comes additional network complexities as multiple sources and destinations are installed at the same physical locations.
The addition of sources and destinations at a single physical location also increases the possibility of injury to technicians installing and terminating fiber optic lines. This is the case because technicians must often use equipment which requires looking into the end of a fiber optic cable. An inadvertent light signal, particularly when magnified, can cause eye injury to the technician if the technician is looking into the fiber optic cable end. In this regard, standards bodies such as the International Telecommunications Union (“ITU”) have promulgated standards such as the ITU-G.664 Automatic Laser Shutdown (“ALS”) standard. In accordance with this standard, a transmitting laser is shutdown when the receiver in the node does not receive a signal from the remote end.
Optical transmission systems typically carry information bit streams as 100% amplitude modulated, also known as on-off keying, signals using an optical carrier signal. As with other carriers in the electromagnetic spectrum, an optical carrier can be modulated many different ways. However, extraction of the information content requires demodulation of the bit stream.
For a fiber optic cable carrying a single optical carrier, a tap may be inserted at any point in the fiber to redirect a portion of the optical carrier power, and the bit stream may be demodulated by a receiver coupled to the tap. This is rarely done for a single carrier fiber. Also, the tap is normally used only to measure DC optical power or a receiver is placed at the extreme end points where no tap is used. In the latter case, the entire optical carrier is terminated at the intended receiver. Dense wave division multiplexing (“DWDM”) systems carry a number of orthogonal optical carriers in a common fiber optic cable. To receive the various bit streams, the carriers must be demultiplexed using wavelength selective filters. The resulting single carrier signals are presented to receivers where the bit stream is demodulated. Once again, optical taps may be present in the fiber optic cable, and often measure the total optical power present in the fiber. This can be done using wide band taps as are known in the art in which there is no frequency selectivity within the band of interest.
A drawback of the wide band tap is that the power of individual wavelengths in the fiber optic cable can not be discerned. Only the aggregate total power can be determined. If there is no modulation scheme other than the original on-off keying (“OOK”), there is no way to determine the individual wavelength power other than by using expensive demultiplexing filters and individual power measuring receivers. Also, the quantity of wavelengths present, and their original source are not discernable using a wide band tap. However, these attributes are useful in systems concerned with fiber interconnect topology path knowledge, mis-fibering, i.e. miswiring detection schemes and trouble-shooting strategies.
It is therefore desirable to be able to determine individual carrier power, individual carrier frequency (or wavelength), and the individual carrier identity present in a multi-carrier fiber optic cable. The identity may be any unique tag which, for example identifies the actual source of a signal.
Systems for determining individual carrier power, individual carrier wavelength, and the source identity are known. For example, sub-carrier modulation schemes such as amplitude modulation (“AM”) schemes have been used.
In a DWDM system using AM technology, each source applies a 100% AM (or OOK) for the bit stream and a low percentage modulation, very low switching rate, signal on each wavelength. The signal is typically a simple sinusoidal tone which periodically switches to a new frequency, for example, every 1.5 seconds. Each optical carrier frequency is assigned a defined set of sub-carrier tones that may be applied to it, for example a set of 8 sub-carrier tones. These tone frequencies form unique sets for each optical carrier frequency. Only one frequency within the set is active at any one time.
Thus, a DWDM fiber optic cable may be observed with a wide band tap in which the receiver is heavily low pass filtered at a cut-off frequency just higher than the highest set of tones which may be present. The frequency domain spectrum of this resultant signal therefore yields a series of spectral lines (or bands), one for each of the active tones in the DWDM aggregate signal. The presence of a tone within a set indicates the presence of a particular wavelength in the fiber optic cable under observation.
The amplitude of the tone applied at the source is a fixed proportion of the actual launch power of the optical carrier. Therefore, the amplitude of the received tone at the monitor point is in direct proportion to the actual optical power received at that point excluding some non-linear effects which are not relevant for discussion or understanding of the present invention. The use of a one-in-a-set of preassigned tone frequencies enables another channel for data communication using a frequency shift keying (“FSK”) approach in which the tone frequency is changed periodically thus representing a symbol time, wherein a tone encodes a multi-bit symbol. For example, in the case of an 8-set of tones, 3 bits of information can be conveyed each symbol time. Thus, an oct-ary FSK modulated bit stream is available. For example, in the case of a typical AM scheme, the tone changes every 1.5 seconds. One bit per symbol is used simply as a clock synchronization indicator, and thus the real effective bit rate is 1.333 bits per second. This FSK modulated stream typically encodes identification information about the actual source for that wavelength, for example, a globally unique serial number.
Under this arrangement, all three pieces of desirable information, wavelength identification, power, and source identification, are available using a wide band tap and receiver in which some simple processing is applied to extract bit streams encoded by each set of tones.
Unfortunately, the scheme described above requires that the optical carrier be continuously active for all attributes to be continuously monitored. However, when fiber optic cables are broken, or during the installation of fiber optic systems, carriers are typically not available in upstream fibers. More importantly, optical power is emitted from the open end of the fiber optic cable, thereby posing a hazard to the service personnel or anyone else who may look at the fiber end at close proximity as described above.
For this reason, the ITU has promulgated the G.664 Standard which executes upon the event of an optical channel being made discontinuous. When the channel is continuous, the optical power is wholly contained in the fiber optic cable and no danger is posed. Also, the G.664 Standard describes the consequent actions to be taken upon discovery of an open optical circuit.
A circuit is considered to be full duplex, consisting of a receiver and transmitter at each end of an optical link . When a fiber strand breaks, the receiver no longer measures an optical power signal at its input. The receiver immediately turns off its related transmitter. The fiber from that transmitter may be geographically distinctly routed compared to the fiber which was cut, and therefore may not also be cut. Nonetheless, as the receiver at the end of that possibly uncut fiber sees no incoming power, it likewise turns off its source. The result is that there is now no optical power being emitted from the dangerous end of the fiber cut. The same occurs if both fibers are cut. This arrangement is referred to as an automatic laser shutdown (“ALS”) operation. Similarly, G.664 also describes an automatic power reduction procedure for amplifiers.
The receivers and the transmitters act automatically to turn off power. It is also desirable to have an automatic procedure to restart the link. The G.664 Standard describes such an ALS restart procedure.
When a source is in an ALS shutdown state, it may emit an optical carrier for 2 seconds±0.25 seconds followed by a fully off period of between 100 and 300 seconds. It is believed that such short pulses of light do not present substantial risk of injury. If a receiver actually observes these pulses, it correctly concludes that the circuit has been re-completed, and it may begin to drive its associated transmitter to carry traffic to its destination.
Although the AM-based identification procedure described above generally requires that an optical carrier be continuously present, because the AM-carrier information is imposed whenever the signal is active. It is possible to convey the frequency, amplitude and FSK modulated identifier/bit stream during ALS pulsing. However, because the bit stream is so low under continuous conditions and because the ALS restart procedure offers the opportunity to convey symbols every 100 to 300 seconds, the very slow conveyance of information is useless (approximately 3 data bits every 100 to 300 seconds). Further, the complexities of maintaining bit synchronization during the 100 to 300 second inactive time is impractical.
It is therefore desirable to be able to convey the source identifier information in an efficient and effective manner even while the ALS restart procedure is in effect.
In a given DWDM bundle, carriers are distinct in the frequency domain. Therefore, there is never a case where a wavelength in a fiber optic cable is not determinable by the frequency tag provided by the AM scheme. Given some high speed control channel such as a common channel which accompanies or is somehow associated with the optical payload such as via an optically-modulated optical-supervisory channel or a separate communications system, it is possible to carry “linked” information such as the source identifier by creating a logical association with the wavelength number. This scheme works perfectly well when there are no re-used frequencies as should be the case in a particular fiber optic cable.
However, in a large office such as may be implemented by a telecommunications carrier, a piece of equipment may serve several DWDM fibers. Each of these fibers may contain any or all of the carrier frequencies available. Thus, in a given office, there may be several distinct sources, each with the same carrier frequency assigned, but meant to serve different fiber optic cables. The possibility of connecting a signal unto the wrong fiber is quite high, and the erroneous case is not distinguishable by examining only the frequency domain match information. The FSK modulated bit stream must be demodulated and compared to the expected value.
Thus, during installation, when ALS will be active, such as is the case where G.664 safety procedures are mandated by the owner or installation personnel, it would take an undesirable amount of time to distinguish the identity of the optical signal source. It is therefore desirable to have a method and system which works in conjunction with ALS while still allowing optical power, the wavelength and source identifier to be easily determined.